UPS mysterious behaviour

Thread Starter


Joined Oct 4, 2011
My UPS is showing abnormal behaviour. The UPS runs almost o.k when 1 fan and around 3 lights are switched on it but as soon as another fan is switched on the first fan starts running faster and the lights go brighter as if output power has been boosted consequently the battery drains faster. What could the possible reason for this be.


Joined Sep 4, 2010
Check the battery condition ....
Some cheaper inverter circuits, like the commercial one I use, have big steps in the front end. Essentially your UPS is probably employing an inductive inverter circuit to generate high voltage DC on a bank of capacitors.
This DC bus is then converted to an approximation of an AC sine wave with a PWM controlled full bridge.

If the DC battery voltage is dipping significantly as the load is increased then the front end inverter will 'change gear' and employ a slightly different strategy to maintain the high voltage DC bus voltage.

In an expensive, 'clever' machine, this change will be smooth so the high voltage DC is closely controlled but many cheaper machines do it in steps which can lead to the phenomena you are seeing.

Of course it could be a many of other issues but checking the basics, are the batteries performing as expected? is a good place to start.

CAUTION ... DO NOT muck about with a UPS or any inverter unless you are absolutely sure you know what you are doing, IT COULD EASILY KILL YOU.

If the batteries can be disconnected do a load test on the pack.
If not call someone who can check them, and the UPS performance, for you.

Your machine may have a display showing its status when running if it dose then you can watch the battery voltage using that.

Hope this helps .... BE VERY CAREFUL